246 



Professor Fleming 



[June 5, 



The platinum temperature by our standard is -81°-9; and corre- 

 sponding to - 182" C, whicli is very nearly the Centigrade tempera- 

 ture of liquid oxygen boiling at the normal pressure of 760 mm. ; 

 the platinum temperature by the same standard is - 197°. The 

 conversion of these numbers representing low temperatures in 

 platinum degrees into the numbers representing the corresponding 

 absolute thermodynamic temperatures is a work we have reserved for 

 a future research ;* but meanwhile it may be said that there is no 

 method of measuring low temperatures which is so easy of applica- 

 tion and so accurate as that depending on the use of a platinum 

 thermometer. All our work has been ultimately referred to one 

 standard platinum thermometer, which we call P^. 



A suggestion may here be made. There is no reason why the Board 

 of Trade electrical laboratory should not possess a standard platinum 

 thermometer defining officially platinum or absolute temperatures for 

 all time, and with which other platinum thermometers could be easily 

 and very accurately compared. 



Having thus defined our scale of temperature, we proceeded to 

 embody the whole of our results in a chart which is now before you 

 (see Fig. 6), and in which vertical distances represent resistivity, or 

 specific resistance, or the resistance in absolute measure per cubic 

 centimetre of the various metals, and horizontal distances represent 

 platinum temperatures. The curves indicate the manner in which 

 the resistivity varies with temperature for each substance. 



The values of the resistivity of most ordinary metals and alloys 

 are given in the table adjoining : — 



Electrical Kesistivity of Puee Annealed Metals. 



Metal. 



Atomic 

 volume. 



Silver .. 

 Copper 

 Gold .. 

 Aluminium 

 Magnesium 

 Zinc ... 

 Iron 



Cadmium 

 Palladium 

 Platinum 

 Nickel . . 

 Tin .. , 

 Thallium , 

 Lead .. 

 Mercury 

 Bismuth 



10-04 



7-10 



10-04 



10-56 



13-76 



9-12 



7-10 



12-96 



9-12 



9-12 



6-94 



16-20 



17-20 



18-27 



14-56 



21-43 



* Callendar has shown that over a wide range of temperature from 0° C. to 

 700° C. the difference between the platinum temperature and the air thermometer 

 temperature is a parabolic function of the absolute temperature. 



